The present invention relates generally to subterranean treatment operations, and more particularly to methods of fracturing sensitive subterranean formations.
In some wells, it may be desirable to individually and selectively create multiple fractures along a well bore at a distance apart from each other. The multiple fractures should have adequate conductivity, so that the greatest possible quantity of hydrocarbons in an oil and gas reservoir can be drained/produced into the well bore. When stimulating a reservoir from a well bore, especially those well bores that are highly deviated or horizontal, it may be difficult to control the creation of multi-zone fractures along the well bore without cementing a liner to the well bore and mechanically isolating the subterranean formation being fractured from previously-fractured formations, or formations that have not yet been fractured.
One conventional method for fracturing a subterranean formation penetrated by a well bore has involved cementing a solid liner in the lateral section of the well bore, performing a conventional explosive perforating step, and then performing fracturing stages along the well bore, using some technique for mechanically isolating the individual fractures. Another conventional method has involved cementing a liner and significantly limiting the number of perforations, often using tightly-grouped sets of perforations, with the number of total perforations intended to create a flow restriction giving a back-pressure of about 100 psi or more; in some instances, the back-pressure may approach about 1000 psi flow resistance. This technology generally is referred to as “limited-entry” perforating technology.
In one conventional method of fracturing, a first region of a formation is perforated and fractured, and a sand plug then is installed in the well bore at some point above the fracture, e.g., toward the heel. The sand plug may restrict any meaningful flow to the first region of the formation, and thereby may limit the loss of fluid into the formation, while a second, upper portion of a formation is perforated and fracture-stimulated. Coiled tubing may be used to deploy explosive perforating guns to perforate subsequent treatment intervals while maintaining well control and sand-plug integrity. Conventionally, the coiled tubing and perforating guns are removed from the well before subsequent fracturing stages are performed. Each fracturing stage may end with the development of a sand plug across the perforations by increasing the sand concentration and simultaneously reducing pumping rates until a bridge is formed. Increased sand plug integrity may be obtained by performing what is commonly known in the cementing services industry as a “hesitation squeeze” technique. A drawback of this technique, however, is that it requires multiple trips to carry out the various stimulation and isolation steps.
The pressure required to continue propagation of a fracture present in a subterranean formation may be referred to as the “fracture propagation pressure.” Conventional perforating operations and subsequent fracturing operations undesirably may cause the pressure to which the subterranean formation is exposed to fall below the fracture propagation pressure for a period of time. In certain embodiments of conventional perforating and fracturing operations, the formation may be exposed to pressures that oscillate above and below the fracture propagation pressure. For example, if a hydrojetting operation is halted temporarily, e.g., in order to remove the hydrojetting tool, or to remove formation cuttings from the well bore before continuing to pump the fracturing fluid, then the formation may experience a pressure cycle.
Pressure cycling may be problematic in sensitive formations. For example, certain subterranean formations may shatter upon exposure to pressure cycling during a fracturing operation, which may result in the creation of numerous undesirable microfractures, rather than one dominant fracture. Still further, certain conventional perforation operations (e.g., perforations performed using wireline tools) often may damage a sensitive formation, shattering it in the area of the perforation so as to reduce the likelihood that subsequent fracturing operations may succeed in establishing a single, dominant fracture.
Similarly, when a subterranean formation is perforated by, e.g., explosive devices, a high amount of compressive force may be imparted to the formation, which may cause a sharp increase (e.g., a “spike”) in pressure. Such pressure spike may significantly damage the formation. To assist in overcoming the damage that may ensue from such pressure spike, techniques such as overbalanced perforating have been employed. In overbalanced perforating, the well bore is pressurized such that the perforation of the formation causes fractures to be formed in the formation at least a short distance from the well bore. However, even techniques such as overbalanced perforating may be problematic, and may lead to pressure cycling of the formation. Often, immediately after the perforation of the formation (e.g., immediately after detonation of the explosive devices), operators may have difficulty increasing the flowrate of fluid to be injected into the formation to an amount sufficient to maintain the fracture, which may cause the well bore pressure to fall, at least briefly, below the fracture propagation pressure. Additionally, where perforation is accomplished by detonation of explosive devices, such explosive devices often are capable of generating substantially greater compressive forces than those that may be reached by injection of a fracturing fluid into the formation. This may result in pressure cycling, as the formation pressure decreases after the initial perforation to a value below the fracture propagation pressure, then increases above the fracture propagation pressure upon injection of the fracturing fluid.